The present invention, in some embodiments thereof, relates to recombinant non-autotrophic microorganisms that are capable of carbon fixation.
Carbon fixation plays an essential role in ecosystems by providing a continuous flux of organic carbon into the biosphere. From a human perspective, the process of CO2 assimilation dominates humanity's usage of land and water. Importantly, under human cultivation, where water, light and nutrients are abundant, carbon fixation often limits plant growth. Therefore, increasing the rate of carbon fixation is of major importance in the path towards agricultural and energetic sustainability.
Carbon fixation in plants, algae, cyanobacteria and many other bacterial lineages is achieved by the Calvin-Benson-Bassham (CBB) Cycle. The productivity of the CBB cycle is limited, under many conditions, by the relatively slow rate and lack of substrate specificity of the carboxylating enzyme RuBisCO. Previous attempts to utilize classic molecular biology tools to improve the rate and specificity of RuBisCO have achieved only limited success. Moreover, several lines of evidence indicate that in spite of its shortcomings, RuBisCO is already naturally optimized given the natural tradeoff between the enzyme's rate and specificity. Other enzymes of the CBB also were shown to constrain carbon fixation in certain circumstances. For example, tobacco plants overexpressing sedoheptulose-1,7-bisphosphatase were characterized by an increased photosynthetic rate and a 30% enhancement in biomass yield. Therefore, improving the rate of the CBB cycle seems to be quite a complex task that has many possible leads. Such a challenge can benefit significantly from the utilization of novel selection systems that can go far beyond the sequence divergence presented in plants.
WO 2011/099006 teaches plants and bacterial cells which are genetically modified so as to express enzymes of alternative (RuBisCO-independent) pathways in order to promote carbon fixation. Autotrophic E. Coli cells which express phosphoribulokinase and Ribulose-Bisphosphate Carboxylase are also disclosed.